There are many situations in which it is desirable to remove a portion of casing from an oil or gas well. Current Oil and Gas UK Guidelines for the Abandonment of Wells (July 2015, Issue 5) dictate that a permanent barrier, typically a cement plug, must be formed between the reservoir and the seabed to act as one of a number of permanent barriers when a well is abandoned or plugged. This measure is intended to isolate the well and reduce the possibility of pressure migration in order to prevent hydrocarbons and other fluids from reservoirs coming to surface and spilling into the sea.
In some situations, prior to installing the cement plug to abandon or plug the well, it may be necessary to remove downhole installations such as production tubing, casing and other downhole tubulars, and the cement or other downhole fixings that secure the downhole installation to the bedrock. In some cases, where cemented casing is used, for example, there may be a leak path in the cement behind the casing or between casing layers. Rectifying such a breach may also require the removal of a casing section and associated cement before forming the cement plug with new cement.
Conventional removal of cemented casing uses, for example, milling tools or hydro-abrasive cutters which remove the metallic casing by gradually cutting or milling away small portions of metal and cement. These are slow processes and therefore make such an operation very expensive and time consuming.